Method of compensating for installation orientation of an attitude determining device onboard a craft

ABSTRACT

In accordance with the disclosed method, an attitude determining device which is installed onboard a mobile craft, like an aircraft, for example, at an unknown orientation with respect to the reference coordinate system of the craft senses its installation orientation with respect to an earth frame coordinate system when the craft is at rest to obtain a static orientation measurement thereof. Thereafter, an attitude of the mobile craft with respect to the earth frame is measured with the attitude determining device and such measurement is compensated with the static orientation measurement to obtain attitude information of the craft&#39;s reference coordinate system with respect to the earth frame coordinate system. The installation orientation of the attitude determining device may be sensed while the craft is at rest in either a leveled or unleveled condition.

BACKGROUND OF THE INVENTION

The present invention relates to attitude determining devices onboard amobile craft for determining the attitude of the craft's referencecoordinate system with respect to an earth frame of reference, and morespecifically, to a method of compensating an attitude measurement ofsuch device for an unknown installation orientation with respect to thereference coordinate system of the craft.

Attitude determining devices for mobile craft, like aircraft, forexample, measure the attitude of the moving craft with respect to anoutside reference coordinate system, typically known as earth frame. Thedevices may be installed at a location in the craft in such a manner tobe mechanically aligned with the reference coordinate system of thecraft. The reference coordinate system of conventional aircraftcomprises three orthogonal axes which include a longitudinal or X axis,a lateral or Y axis, and a vertical or Z axis. Motion of the aircraft isgenerally described as roll which is a rotation about the X axis, pitchwhich is a rotation about the Y axis and yaw which is a rotation aboutthe Z axis. Pitch, roll and yaw positions are measured as the currentangle between the aircraft reference coordinate system and earth frame.Conventionally, aircraft attitude determining devices primarily measureattitude of the aircraft in pitch and roll.

Any inaccuracy in installing an attitude determining device in the craftwith respect to the reference coordinate system thereof will result ininaccurate measurement and presentation of the attitude of the craft toeither the pilot or other system using the attitude information fordisplay or control purposes. Currently, a method of installing thesedevices in an aircraft has been to accurately level the aircraft first,and then, install the device using shims or other mechanical apparatusto correctly position the device with respect to the three orthogonalaxes forming the coordinate system of the aircraft. This procedure ofleveling is adequate for devices mounted in locations of the aircraftremote from the cockpit, but when the device is to be mounted in acockpit location, such as on an instrument panel, for example, shimmingor other mechanical means of adjusting the installation orientationthereof may be precluded due to viewing angle restrictions, aesthetics,. . . etc. Accordingly, some other compensation method will be required.

Currently, units installed on an instrument panel in the cockpit of anaircraft have slots for roll axis alignment and internal mechanicalmeans to accommodate pitch angles other than zero. However, theseaccommodations for pitch angles make the assumption of zero error inmanufacturing tolerances of the aircraft panel angle.

Accordingly, the inventive method described herein below ensures asubstantially accurate measurement of aircraft attitude by the attitudedetermining device with respect to the earth frame of reference. Thestatic installation orientation is automatically determined by thedevice itself and the attitude measurement is compensated therewith in aprocessor of the device. Thus, the drawbacks of the current mechanicalleveling and alignment procedures are avoided.

SUMMARY OF THE INVENTION

In accordance with the present invention, an attitude determining devicewhich is installed onboard a mobile craft at an unknown orientation withrespect to the reference coordinate system of the craft senses itsinstallation orientation with respect to an earth frame coordinatesystem when the craft is at rest to obtain a static orientationmeasurement. An attitude of the mobile craft is measured with theattitude determining device and such measurement is compensated with thestatic orientation measurement to obtain attitude information of thecraft's reference coordinate system with respect to the earth framecoordinate system.

In one embodiment, the acceleration of the attitude determining deviceis sensed for each of the axes of the reference coordinate system of themobile craft while at rest and leveled, and a static attitude pitch andstatic attitude roll of the device are determined from trigonometricfunctions of ratios of the sensed accelerations. Accordingly, both ofthe measured attitude pitch and roll of the device are compensated withthe static attitude pitch and the static attitude roll, respectively, inthe attitude determining device to render attitude information of thecraft's reference coordinate system with respect to the earth framecoordinate system.

In another embodiment, a static attitude of the mobile craft in pitchand roll is obtained while the craft is at rest and unleveled.Thereafter, the static attitude craft pitch is used in determining thestatic attitude pitch of the device and the static attitude craft rollis used in determining the static attitude roll of the device and suchstatic attitude pitch and roll are used respectively to compensate forthe measured attitude pitch and roll of the mobile craft in the attitudedetermining device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an aircraft, with it's reference coordinatesystem, onboard which an attitude determining device may be installed.

FIG. 2 is an illustration of an attitude determining device includingconventional internal acceleration and rate sensors for three orthogonalaxes X, Y and Z.

FIG. 3 is a sketch of an attitude determining device mounted on a panelin the cockpit of an aircraft at an unknown orientation to the referencecoordinate system of the craft.

FIGS. 4A and 4B are illustrations exemplifying methods of determiningthe pitch and roll of the attitude determining device onboard a mobilecraft using sensed acceleration measurements of the device in accordancewith the present invention.

FIG. 5 is a block diagram schematic representing a suitable embodimentof an attitude determining device for performing the method inaccordance with the present invention.

FIG. 6 is a block diagram schematic representing an alternate embodimentof an attitude determining device for performing another aspect of thepresent invention.

FIG. 7 is an illustration of an aircraft having its reference coordinatesystem unleveled with respect to an earth frame coordinate systemallowing for offset angles of pitch and roll respectively from a levelattitude.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the present embodiment, an aircraft will be used, by way of example,as a mobile craft, but it is understood that other similar craft may beused where ever an attitude of the craft is desired and measured withrespect to an earth frame of reference coordinate system, hereinafterreferred to simply as earth frame. An aircraft with its referencecoordinate system is shown in FIG. 1 including a longitudinal axisdepicted as an X axis, a lateral axis depicted as a Y axis, and avertical axis depicted as a Z axis. Accordingly, roll of the aircraftmay be measured as the angular rotation about the X axis, pitch of theaircraft may be measured by the angular rotation about the Y axis andyaw of the aircraft may be measured by the angular rotation about thevertical Z axis. All of these angles are measured with respect to theearth frame. Conventionally, an attitude determining device of anaircraft measures attitude in pitch and roll.

To accurately level the aircraft 10 such that its reference coordinateaxes coincides with earth frame, the aircraft is adjusted in attitudesuch that an acceleration a_(z) sensed for the Z axis is setsubstantially equal to a gravity vector g, and the accelerations sensedin the X axis, a_(x) and in the Y axis, a_(y), are set substantially tozero. When these conditions are sensed and stabilized, the aircraft 10is considered leveled.

FIG. 2 is an illustration of an attitude determining device 20 which mayinclude conventional internal acceleration sensors for the threeorthogonal axes X, Y and Z, and may also include conventional ratesensors to measure the rotational motion ω_(x), ω_(y), and ω_(z) whichare the rotational motions about the respective axes X, Y and Z. Anexample of such a device is an inertial reference unit manufactured byHoneywell, Inc., model no. HG2001AB02. The internal acceleration sensors(not shown) determine the gravity vector or local vertical g.Thereafter, rotational motion about the respective axes X, Y and Z issensed by the rate sensors (also not shown), the output of which beingintegrated over time to maintain a real time craft attitude. Anyaccumulated integration errors may be removed during static periods byre-aligning the derived output of the device to the local vertical gwhich procedure is referred to as leveling or erection. Thesecalculations are conventionally performed by a processor internal to thedevice which samples the sensor outputs and performs the initial andcontinuous algorithms to produce an attitude solution to be used fordisplay in the aircraft or for a guidance and/or control application forthe aircraft.

The attitude determining device 20 may be of a strap down system whichis mechanically mounted to the case of the device or a gimballedinstrument having elements which are free to rotate in inertial spaceindependent of the case of the unit. In either case, in locating theattitude determining device 20 on board a moving craft, like anaircraft, for example, it may be installed at an unknown orientationwith respect to the reference coordinate system of the craft which inthe present embodiment are the three orthogonal axes X, Y and Z. It isdesired that the device be mounted level with the lateral andlongitudinal axes of the craft and aligned with the longitudinal X axissuch as shown in FIG. 2, but this may not always be possible due toerrors in mechanical leveling or adjusting of the orientation and due toerrors in manufacturing tolerances of the device and the aircraftstructure where the device is being mounted. This is especially evidentwhen the attitude determining device 20 is mounted on a panel in thecockpit of the aircraft 10 much as illustrated in the sketch of FIG. 3.

Referring to FIG. 3, when the attitude determining device 20 isinstalled on an aircraft instrument panel 30, the device may not bealigned with the "waterline" or level line of the aircraft in order tocompute accurate attitude information. This is because the panel isoften not perpendicular to the waterline and it is not possible in mostcases to exactly compensate mechanically for the panel angle offset 40to the vertical or Z axis. In accordance with the present invention, amethod is described below which ensures an accurate calculation of theattitude of a moving craft, like an aircraft, for example, by measuringthe installation orientation of the device 20 with respect to thereference coordinate axes of the aircraft and compensating for thisorientation mathematically in a processor of the device 20.

In the present embodiment, upon installation of the device 20 on theinstrumentation panel 30 of the craft 10, whose reference coordinateaxes have been leveled to coincide with earth frame, the installationorientation thereof is automatically measured by the installed device 20and stored in a non-volatile memory thereof. For example, the pitch,φ_(c) and the roll, ρ_(c) are measured using the acceleration sensors ofthe device 20 and this measurement is exemplified by the illustrationsof FIGS. 4A and 4B. In FIG. 4A, the panel 30 and mounted device 20 isshown in the plane of the axes Z and Y to describe the measurement ofthe roll angle ρ_(c) of the installed device 20. In the plane of theaxes Z and Y, the acceleration vectors a_(y) and a_(z) are addedvectorially to yield the gravity vector g. The installation roll angleρ_(c) about the X axis is the angle between the vectors g and a_(z) andmay be determined mathematically in accordance with a trigonometricfunction of the ratio of a_(y) to a_(z).

Similarly, the perspective of the device 20 installed on the panel 30 inthe plane of the axes X and Z is shown in FIG. 4B. Referring to FIG.4B,in this perspective, the acceleration vectors a_(z) and a_(x) add upvectorially to yield the gravity vector g and the pitch angle φ_(c) isthe angle between the vectors g and a_(z) which is a rotation about theY axis. The installation pitch angle φ_(c) may be determinedmathematically in accordance with a trigonometric function of the ratioof a_(x) to a_(z). In the present embodiment, the trigonometric functionused for determining the installed roll and pitch angles for staticorientation of the device 20 is the arcsine.

A block diagram schematic representing a suitable embodiment of theattitude determining device is shown in FIG. 5. Referring to FIG. 5,after the device is installed on the instrument panel of a leveled craft10, for example, and power is subsequently activated to the device 20,an internal processor 52 of the device 20 samples the outputs of theacceleration senors depicted in the block 50 in all three axesa_(x),a_(y),a_(z). The static angles of the device 20 with respect toearth frame are determined by the processor 52 from the staticacceleration measurements based on the trigonometric function describedabove. The installation angles φ_(c) and ρ_(c) are read fromnon-volatile memory 54 of the device 20 and the attitude of the craft 10with respect to earth frame is determined by the processor 52 bysubtracting these installation angles φ_(c) and ρ_(c) from the staticangles of the device 20 with respect to earth frame. Thereafter, thepitch and roll attitude angles of the moving craft 10 are computedconventionally by the processor 52 via the rate sensors ω_(x), ω_(y)ω_(z) which are shown at block 56 of the device 20 and received by theprocessor 52. In a gimballed attitude determining device the angles ofthe spin axis, measured using synchros or other such devices, withrespect to the case are corrected by subtracting the installation anglesφ_(c) and ρ_(c) to yield actual aircraft pitch and roll attitude angles.

In summary, for the case in which the craft is leveled according to thedescription supplied above prior to sensing the installation orientationof device 20, the processor 52 samples the outputs a_(x),a_(y) and a_(z)of the acceleration sensors 50. The static installation angles φ_(c) andρ_(c) are determined by the processor 52 from the static accelerationmeasurements based on the trigonometric function described above and arestored in a non-volatile memory 54 for use in compensating the attitudemeasurements with respect to earth frame. Power to device 20 may then beremoved. Subsequent power application to device 20 would allow ameasurement of the attitude of the aircraft, i.e. orientation of theaircraft's reference coordinate axes with respect to earth frame, to becorrectly determined by processor 52 using φ_(c) and ρ_(c) from thememory 54.

In some applications, the attitude determining device 20 may not includeacceleration sensors 50 but rather include level sensors for sensingdirectly the pitch φ_(case) and roll ρ_(case) of the installed case withrespect to the earth frame. A block diagram schematic suitable forexemplifying an alternate embodiment of the device 20 including levelsensors is shown in FIG. 6 with the level sensing depicted at 58. Likereference numerals are given to the other elements of the device 20 tomatch those described in connection with the embodiment of FIG. 5. Inoperation, the processor receives the installation orientation anglesφ_(case) and ρ_(case) measured by the level sensors at 58 and storesthem in the non-volatile memory 54 as φ_(c) and ρ_(c) to be accessedsubsequently in compensating for the attitude angle measurements asdescribed in connection with the embodiment of FIG. 5.

The foregoing method provides for compensating for the installationorientation of the device 20 for a leveled craft. If the craft 10 is notin a level attitude as shown in the exemplified illustration of FIG. 7,the actual unlevel aircraft attitude may be measured i.e. referencecoordinate axes of the aircraft with respect to earth frame, allowingthe processor 52 to determine the offset angles of pitch and roll, φ_(e)and ρ_(e), respectively, from a level attitude. These pitch and rollangle offsets from a level condition of the aircraft may be input eithermanually or electrically to the processor 52 of the device 20 as shownin FIGS. 5 and 6. In addition, the static installation angles aremeasured by device 20 with respect to the unleveled aircraft coordinateaxes. In order for the processor 52 of device 20 to calculate theeffective static installation pitch and roll angles, φ_(c) and ρ_(c) ofthe case with respect to a level reference coordinate system of thecraft 10, it may subtract the measured offset angles from theirrespective measured installation angles. The effective staticorientation measurements of the case with respect to the craft'sreference coordinate system may then be stored in the memory 54 as shownin FIGS. 5 and 6 in order to compensate for the installation orientationof the device in the craft 10 as described supra.

In attitude determining devices in which there is no non-volatilememory, the step of sensing the installation orientation of the deviceto obtain a static orientation measurement with respect to the referencecoordinate system of the craft may be performed each time the power isturned on and the aircraft is in a static condition. The resultingstatic orientation measurement may be stored in the memory of the devicefor use in compensating for attitude measurements for the moving craft.

While the invention has been described herein in connection with apreferred embodiment, it should not be so limited, but rather construedin accordance with the breath and broad scope of the claim set appendedhereto.

We claim:
 1. A method of compensating for installation orientation of anattitude determining device on-board a mobile craft with respect to areference coordinate system of said craft to obtain attitude informationof said craft from said device based on an earth frame coordinatesystem, said method comprising the steps of:installing said attitudedetermining device on-board said mobile craft at an unknown orientationwith respect to said reference coordinate system of said craft; sensingthe installation orientation of said attitude determining device withrespect to said earth frame coordinate system when said craft is at restto obtain a static orientation measurement of said device; measuring anattitude of said mobile craft with said attitude determining device; andcompensating said craft attitude measurement of said device with saidstatic orientation measurement to obtain attitude information of saidcraft's reference coordinate system with respect to said earth framecoordinate system.
 2. The method in accordance with claim 1 wherein thereference coordinate system of said craft includes three orthogonalaxes--a vertical or z axis, a longitudinal or x axis and a lateral or yaxis.
 3. The method in accordance with claim 2 wherein the step ofsensing includes:leveling the craft while at rest such that the z axisis aligned with a gravity vector and no substantial at rest accelerationexists at the x and y axes; sensing the acceleration at the device foreach of said three axes--a(x), a(y) and a(z) while the craft is at restand leveled; and determining the static orientation measurement of saiddevice based on a function of said three sensed axisaccelerations--a(x), a(y) and a(z).
 4. The method in accordance withclaim 3 wherein the step of determining includes:determining a staticattitude pitch of the device as a trigonometric function of a ratio ofthe sensed accelerations a(x) and a(z); and determining a staticattitude roll of the device as a trigonometric function of a ratio ofthe sensed accelerations a(y) and a(z); and wherein the staticorientation measurement of the device comprises the determined staticattitude pitch and static attitude roll.
 5. The method in accordancewith claim 4 wherein the step of measuring includes measuring anattitude pitch and an attitude roll of the mobile craft with saiddevice; and the step of compensating includes compensating the measuredattitude pitch with the static attitude pitch and compensating themeasured attitude roll with the static attitude roll.
 6. The method inaccordance with claim 2 wherein the step of sensing includes:sensing theacceleration at the device for each of said three axes--a(x), a(y) anda(z) while the craft is at rest and unleveled; obtaining a staticattitude of the craft while at rest and unleveled; determining thestatic orientation measurement of said device based on said static craftattitude and a function of said three sensed axis accelerations--a(x),a(y) and a(z).
 7. The method in accordance with claim 6 wherein the stepof obtaining includes:obtaining a static craft pitch and a static craftroll; and the step of determining includes:determining a static attitudepitch of the device as a trigonometric function of a ratio of the sensedaccelerations a(x) and a(z) and said static craft pitch; and determininga static attitude roll of the device as a trigonometric function of aratio of the sensed accelerations a(y) and a(z) and said static craftroll; and wherein the static orientation measurement of the devicecomprises the determined static attitude pitch and static attitude roll.8. The method in accordance with claim 7 wherein the step of measuringincludes measuring an attitude pitch and an attitude roll of the mobilecraft with said device; and the step of compensating includescompensating the measured attitude pitch with the static attitude pitchand compensating the measured attitude roll with the static attituderoll.
 9. The method in accordance with claim 1 wherein the step ofsensing includes:leveling the craft while at rest; sensing aninstallation pitch and an installation roll of the device while thecraft is at rest and leveled; and wherein the static orientationmeasurement of the device comprises the sensed installation pitch androll of the device.
 10. The method in accordance with claim 9 whereinthe step of measuring includes measuring an attitude pitch and anattitude roll of the mobile craft with said device; and the step ofcompensating includes compensating the measured attitude pitch with thesensed installation pitch and compensating the measured attitude rollwith the sensed installation roll.
 11. The method in accordance withclaim 1 wherein the step of sensing includes:sensing an installationpitch and an installation roll of the device while the craft is at restand unleveled; obtaining a static attitude pitch and a static attituderoll of the craft while at rest and unleveled; determining a staticattitude pitch of said device based on a combination of said staticcraft attitude pitch and said installation pitch and a static attituderoll of the device based on a combination of said static craft attituderoll and said installation roll; wherein the static orientationmeasurement of the device comprises the determined static deviceattitude pitch and static device attitude roll.
 12. The method inaccordance with claim 11 wherein the step of measuring includesmeasuring an attitude pitch and an attitude roll of the mobile craftwith said device; and the step of compensating includes compensating themeasured attitude pitch with the static device attitude pitch andcompensating the measured attitude roll with the static device attituderoll.
 13. The method in accordance with claim 1 wherein the mobile craftis an aircraft, and the attitude device is installed on aninstrumentation panel of said aircraft.
 14. The method in accordancewith claim 1 wherein the attitude determining device comprises astrapdown attitude instrument.
 15. The method in accordance with claim 1wherein the attitude determining device comprises a gimballed attitudeinstrument.
 16. A method of compensating for installation orientation ofan attitude determining device on-board a mobile craft with respect to areference coordinate system of said craft to obtain attitude informationof said craft from said device based on an earth frame coordinatesystem, said method comprising the steps of:installing said attitudedetermining device on-board said mobile craft at an unknown orientationwith respect to said reference coordinate system of said craft; sensingthe installation orientation of said attitude determining device withrespect to said earth frame coordinate system when said craft is at restto obtain a static orientation measurement of said device; storing saidstatic orientation measurement in a memory; measuring an attitude ofsaid mobile craft with said attitude determining device; retrieving saidstatic orientation measurement from said memory to a processor of saiddevice; and compensating said craft attitude measurement with saidretrieved static orientation measurement in said processor to obtainattitude information of said craft's reference coordinate system withrespect to said earth frame coordinate system.
 17. The method inaccordance with claim 16 wherein the step of sensing includes:sensingthe installation orientation of the device with sensors disposed at thedevice; receiving in the processor sensed orientation data of saidsensors; and processing the received data in the processor to obtain thestatic orientation measurement of the device.
 18. The method inaccordance with claim 17 wherein the step of sensing includes sensingthe installation orientation of the device with acceleration sensors.19. The method in accordance with claim 17 wherein the step of sensingincludes sensing the installation orientation of the device with levelsensors.
 20. The method in accordance with claim 16 wherein the step ofcompensating includes:obtaining a static attitude of the craft while atrest and unleveled; providing said static craft attitude to theprocessor of said device; and compensating said craft attitudemeasurement with said retrieved static orientation measurement andstatic craft attitude in said processor to obtain attitude informationof the craft's reference coordinate system with respect to the earthframe coordinate system.